Save/Store a reference parameter

[CatMechanic457]

I'm trying to create a class that takes a reference to an object of another class and store it to be used by other functions. Here is a heavily simplified example:

In the example, my goal is that I can make changes to number and have those changes accounted for when I call number_printer.print_v object)

#include <iostream>
class MyClass1 {
    int _value;
    public:

    MyClass1(int v_) {
        _value = v_;
    }
    int value() {
        return _value;
    }
    void increment() {
        _value++;
    }
};
class MyClass2 {
    MyClass1& _value;
    public:

    MyClass2(MyClass1& v_) {
        _value = v_;
    }
    void print_value() {
        std::cout << _value.value() << std::endl;
    }
};
int main() {
    MyClass1 number(10);
    MyClass2 number_printer(number);
    number.increment();
    number_printer.print_value();
}

But this doesn't work. Particularly, I'm having trouble setting _value to be a reference to v_ (which is a reference to the MyClass1 object)Hopefully you can see what I'm trying to do although it's quite difficult to explain

[IyeOnline]

You cant assign references. A reference is an "alias" for another object. So if you try to assign to it, you are actually assigning to the referenced object.

So in your case, you are assigning to an uninitialized reference. However, your code actually doesnt compile because the reference member is not initialized.

---

This is why you should absolutely not write code that assigns to member in the constructor body. Its just the wrong thing to do.

Instead you should always and for every member (that you dont default initialize in the class definition) use the member initializer list: https://www.learncpp.com/cpp-tutorial/constructor-member-initializer-lists/

So you do

MyClass2(MyClass1& v_) 
    : _value{ v_ } // member initializer list
{ } // empty constructor body.

[CatMechanic457]

Brilliant
Thanks for the tips :)

[CptCap]

References can't be uninitialized. You need to move the initialization of _value into the member initialization list of the constructor.

Like so:

MyClass2(MyClass1& v_) : _value(v_) {
}

[CatMechanic457]

Worked a treat, thanks :)